From the state of the art, an anti-blocking control system, an anti-slip control system, a braking force distribution system and a driving dynamic control system are known which control the action of the brakes and/or drives to the individual wheels or individual vehicle axles. An important influence quantity is, inter alia, the rpm of the individual wheels. With a reduction of the diameter of a wheel (for example, by the mounting of a so-called emergency wheel or by a loss in air pressure), erroneous computations which affect safety can occur in such systems because of an incorrect wheel rpm detection. Emergency wheels have, as a rule, a circumference which is up to 25% less than the normal wheels.
From the state of the art, systems for detecting the condition of tires or wheels are known in many variations. Here, of special significance is the detection of a wheel or tire diameter which deviates from the other wheels. Such a deviation can either be caused by a loss in tire air or by a substitute wheel having a deviating diameter, for example, an emergency wheel.
DE-OS 36 30 116 and DE-PS 32 36 520 describe arrangements for indicating the condition of tires of a vehicle wherein the differences of the rotational speeds of individual wheels are determined at specific operating conditions (unbraked, unaccelerated straight-line travel). It is especially suggested to standardize these rotational speeds to the particular vehicle road speed.
A tire tolerance compensation is described in DE-OS 41 13 278 wherein ratios of the rpm speeds of wheels on respective sides of the vehicle are formed in order to compensate the wheel rpms and corrected values for compensating are derived therefrom.
For an anti-blocking control system, a system for detecting an emergency wheel is described in EP,B1,0 449 845 wherein the slip values of the wheels are determined from the wheel rotational speeds and the estimated vehicle road speed (reference speed). If a slip value increases above a threshold value for a specific time duration, then the presence of an emergency wheel is detected and the anti-blocking controller is switched off.